Time Distribution Switch

ABSTRACT

Systems and methods for detecting the failure of a precision time source using an independent time source are disclosed. Additionally, detecting the failure of a GNSS based precision time source based on a calculated location of a GNSS receiver is disclosed. Moreover, the system may be further configured to distribute a time derived from the precision time source as a precision time reference to time dependent devices. In the event of a failure of the precision time source, the system may be configured to distribute a time derived from a second precision time source as the precision time signal during a holdover period.

RELATED APPLICATION

The present application claims the benefit under 35 U.S.C. §119(e) ofU.S. Provisional Patent Application No. 61/716,310 filed 19 Oct. 2012and titled “Time Distribution Switch” which application is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

This disclosure relates to detecting the failure of a precision timesource using an independent time source. Particularly, this disclosurerelates to detecting the failure of a precision time source in anelectric power transmission or distribution system.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the disclosure aredescribed, including various embodiments of the disclosure withreference to the figures, in which:

FIG. 1 is a one-line diagram of an electric power delivery system.

FIG. 2 illustrates a time distribution system including communicationsIEDs configured to distribute a precision time reference to variousIEDs.

FIG. 3 illustrates an embodiment of a time distribution deviceconfigured to receive, distribute, and/or determine a precision timereference.

FIG. 4 illustrates one embodiment for determining whether a primary orbest available time source has failed.

FIG. 5 illustrates another embodiment for determining whether a primaryor best available time source has failed.

FIG. 6 illustrates one embodiment for determining whether a primary orbest available time source has failed based on GNSS location.

FIG. 7 illustrates one embodiment for determining a best available timesource.

FIG. 8 illustrates an example diagram depicting the variation of theduration between consecutive time signals.

In the following description, numerous specific details are provided fora thorough understanding of the various embodiments disclosed herein.However, those skilled in the art will recognize that the systems andmethods disclosed herein can be practiced without one or more of thespecific details, or with other methods, components, materials, etc. Inaddition, in some cases, well-known structures, materials, or operationsmay not be shown or described in detail in order to avoid obscuringaspects of the disclosure. Furthermore, the described features,structures, or characteristics may be combined in any suitable manner inone or more alternative embodiments.

DETAILED DESCRIPTION

Electric power transmission and distribution systems may utilizeprecision time information to perform various monitoring, protection,and communication tasks. In connection with certain applications,intelligent electronic devices (IEDs) and network communication devicesmay utilize time information accurate beyond the millisecond range. IEDswithin a power system may be configured to perform metering, control,and protection functions that require a certain level of precisionbetween one or more IEDs. For example, IEDs may be configured tocalculate and communicate time-synchronized phasors (synchrophasors),which may require that the IEDs and network devices be synchronized towithin nanoseconds of one other. Many protection, metering, control, andautomation algorithms used in power systems may benefit from or requirereceipt of precision time information.

Various systems may be used for distribution of precision timeinformation. According to various embodiments disclosed herein, a powersystem may include components connected using a synchronized opticalnetwork (SONET). In such embodiments, precision time information may bedistributed using a synchronous transport protocol and synchronoustransport modules (STMs). According to one embodiment, a precision timereference can be transmitted within a frame of a SONET transmission. Inanother embodiment, a precision time reference may be incorporated intoa header or an overhead portion of a SONET STM frame. Similarly, thepower system may include components connected using Synchronous DigitalHierarchy (SDH) protocol. Although several embodiments herein aredescribed in terms of SONET, it should be recognized that the SDHprotocol may be used in place of SONET unless otherwise specified.

IEDs, network devices, and other devices in a power system may includelocal oscillators or other time sources and may generate a local timesignal. In some circumstances, however, external time signals, providedby a time distribution device, may be more precise and may therefore bepreferred over local time signals. A power system may include a datacommunications network that transmits a precision time reference fromthe time distribution device to time dependent devices connected to thedata communications network. In some embodiments, the communicationsnetwork may include one or more local area networks (LANs) and one ormore wide area networks (WANs). In a system with multiple LANs, multipletime distribution devices (one or more for each LAN) may be connected tothe data communications network and each time distribution device canprovide a precision time reference to other time distribution devicesacross the WAN. In each time distribution device, the precision timereference may be received or derived from an external precision timesignal.

According to various embodiments, each time distribution device receivesmultiple precision time signals from various time sources and isconfigured to provide the best available precision time signal as theprecision time reference. The precision time signals may be receivedusing an Inter-Range Instrumentation Group (IRIG) protocol, a globalnavigation satellite system (GNSS, such as, for example, globalpositioning system (GPS), GLONASS, or the like), a radio broadcast suchas a National Institute of Science and Technology (NIST) broadcast(e.g., radio stations WWV, WWVB, and WWVH), the IEEE 1588 protocol, anetwork time protocol (NTP) codified in RFC 1305, a simple network timeprotocol (SNTP) in RFC 2030, and/or another time transmission protocolor system.

While, the above listed precision time signals may provide accurate timeto a time distribution device, they vary in quality. For example, theprecision of NTP and SNTP is limited to the millisecond range, thusmaking it inappropriate for sub-millisecond time distributionapplications. Further, both protocols lack security and are susceptibleto malicious network attacks. The IEEE 1588 standard includeshardware-assisted timestamps, which allows for time accuracy in thenanosecond range. Such precision may be sufficient for more demandingapplications (e.g., the sampling of the sinusoidal currents and voltageson power lines to calculate “synchrophasors”). It is well suited fortime distribution at the communication network periphery, or amongindividual devices within the network. GNSS time signals provide a veryaccurate and robust time measurement, however GNSS signals aresusceptible to spoofing. Therefore, it would be beneficial to provide asystem and method for detecting failure in any of the received precisiontime signals such that the best available precision time reference canbe provided to time dependent devices.

In certain embodiments, when the time distribution device determinesthat the connection to the best available time source has failed, a newbest available time source may be selected from the remaining availabletime sources. In addition to relying on a precision time reference fromthe time distribution device, when available, the various time dependentdevices may be configured to enter a holdover period when the precisiontime reference is unavailable. In some embodiments, a device may beconfigured to monitor the drift of a local time source with respect tothe precision time reference and to retain information regarding thedrift. During the holdover period, an IED or network device may rely ona local time signal.

Reference throughout this specification to “one embodiment” or “anembodiment” indicates that a particular feature, structure, orcharacteristic described in connection with the embodiment is includedin at least one embodiment. Thus, the appearances of the phrases “in oneembodiment” or “in an embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.In particular, an “embodiment” may be a system, an article ofmanufacture (such as a computer readable storage medium), a method, anda product of a process.

The phrases “connected to,” “networked,” and “in communication with”refer to any form of interaction between two or more entities, includingmechanical, electrical, magnetic, and electromagnetic interaction. Twocomponents may be connected to each other even though they are not indirect physical contact with each other and even though there may beintermediary devices between the two components.

Some of the infrastructure that can be used with embodiments disclosedherein is already available, such as: general-purpose computers,computer programming tools and techniques, digital storage media, andoptical networks. A computer may include a processor such as amicroprocessor, microcontroller, logic circuitry, or the like. Theprocessor may include a special purpose processing device such as anASIC, PAL, PLA, PLD, Field Programmable Gate Array, or other customizedor programmable device. The computer may also include a computerreadable storage device such as non-volatile memory, static RAM, dynamicRAM, ROM, CD-ROM, disk, tape, magnetic, optical, flash memory, or othercomputer readable storage medium.

As used herein, the term IED may refer to any microprocessor-baseddevice that monitors, controls, automates, and/or protects monitoredequipment within the system. Such devices may include, for example,remote terminal units, differential relays, distance relays, directionalrelays, feeder relays, overcurrent relays, voltage regulator controls,voltage relays, breaker failure relays, generator relays, motor relays,automation controllers, bay controllers, meters, recloser controls,communications processors, computing platforms, programmable logiccontrollers (PLCs), programmable automation controllers, input andoutput modules, and the like. IEDs may be connected to a network, andcommunication on the network may be facilitated by networking devicesincluding, but not limited to, multiplexers, routers, hubs, gateways,firewalls, and switches. Furthermore, networking and communicationdevices may be incorporated in an IED or be in communication with anIED. The term IED may be used interchangeably to describe an individualIED or a system comprising multiple IEDs.

IEDs, network devices, and time distribution devices may be physicallydistinct devices, may be composite devices, or may be configured in avariety of ways to perform overlapping functions. IEDs, network devices,and time distribution devices may comprise multi-function hardware(e.g., processors, computer-readable storage media, communicationsinterfaces, etc.) that can be utilized in order to perform a variety oftasks, including tasks typically associated with an IED, network device,and/or time distribution device. For example, a network device, such asa multiplexer, may also be configured to issue control instructions to apiece of monitored equipment. In another example, an IED may beconfigured to function as a firewall. The IED may use a networkinterface, a processor, and appropriate software instructions stored ina computer-readable storage medium in order to simultaneously functionas a firewall and as an IED. In another example, an IED may include thenecessary hardware and software instructions to function as a timedistribution device for other IEDs in a LAN or WAN. In order to simplifythe discussion, several embodiments disclosed herein are illustrated inconnection with time distribution devices; however, one of skill in theart will recognize that the teachings of the present disclosure,including those teachings illustrated only in connection with timedistribution devices, are also applicable to IEDs and network devices.

Aspects of certain embodiments described herein may be implemented assoftware modules or components. As used herein, a software module orcomponent may include any type of computer instruction or computerexecutable code located within a computer readable storage medium. Asoftware module may, for instance, comprise one or more physical orlogical blocks of computer instructions, which may be organized as aroutine, program, object, component, data structure, etc., that performsone or more tasks or implements particular abstract data types.

In certain embodiments, a particular software module may comprisedisparate instructions stored in different locations of a computerreadable storage medium, which together implement the describedfunctionality of the module. Indeed, a module may comprise a singleinstruction or many instructions, and may be distributed over severaldifferent code segments, among different programs, and across severalcomputer readable storage media. Some embodiments may be practiced in adistributed computing environment where tasks are performed by a remoteprocessing device linked through a communications network. In adistributed computing environment, software modules may be located inlocal and/or remote computer readable storage media. In addition, databeing tied or rendered together in a database record may be resident inthe same computer readable storage medium, or across several computerreadable storage media, and may be linked together in fields of a recordin a database across a network.

The software modules described herein tangibly embody a program,functions, and/or instructions that are executable by computer(s) toperform tasks as described herein. Suitable software, as applicable, maybe readily provided by those of skill in the pertinent art(s) using theteachings presented herein and programming languages and tools, such asXML, Java, Pascal, C++, C, database languages, APIs, SDKs, assembly,firmware, microcode, and/or other languages and tools.

A precision time reference refers to a time signal or time source reliedon by a plurality of devices and distributed by a time distributiondevice, and which is presumed to be more precise than a local timesource. The determination of accuracy may be made based upon a varietyof factors. A precision time reference may allow for specific moments intime to be described and temporally compared to one another.

A time source is any device that is capable of tracking the passage oftime. A variety of types of time sources are contemplated, including avoltage-controlled temperature compensated crystal oscillator (VCTCXO),a phase locked loop oscillator, a time locked loop oscillator, arubidium oscillator, a cesium oscillator, a trained oscillator, amicroelectromechanical device (MEM), and/or other device capable oftracking the passage of time.

A time signal is a representation of the time indicated by a timesource. A time signal may be embodied as any form of communication forcommunicating time information. A wide variety of types of time signalsare contemplated, including an Inter-Range Instrumentation Group (IRIG)protocol, a global navigation satellite system (GNSS, such as, forexample, global positioning system (GPS), GLONASS, or the like), a radiobroadcast such as a National Institute of Science and Technology (NIST)broadcast (e.g., radio stations WWV, WWVB, and WWVH), the IEEE 1588protocol, a network time protocol (NTP) codified in RFC 1305, a simplenetwork time protocol (SNTP) in RFC 2030, and/or another timetransmission protocol or system. Time source and time signal may be usedinterchangeably herein.

Failure of a precision time source and/or precision time signal, as usedherein, includes spoofing and/or jamming the signal, mechanical orsoftware failures, system wide outages, etc.

Furthermore, the described features, operations, or characteristics maybe combined in any suitable manner in one or more embodiments. It willalso be readily understood that the order of the steps or actions of themethods described in connection with the embodiments disclosed hereinmay be changed, as would be apparent to those skilled in the art. Thus,any order in the drawings or detailed description is for illustrativepurposes only and is not meant to imply a required order, unlessspecified to require an order.

FIG. 1 illustrates a one-line diagram of an electric power deliverysystem 10. The delivery system 10 includes intelligent electronicdevices (IEDs) 102, 104, and 106 utilizing a precision time reference tomonitor, protect, and/or control system components. The electric powertransmission and delivery system 10 illustrated in FIG. 1 includes threegeographically separated substations 16, 22, and 35. Substations 16 and35 include generators 12 a, 12 b, and 12 c. The generators 12 a, 12 b,and 12 c generate electric power at a relatively low voltage, such as 12kV. The substations include step-up transformers 14 a, 14 b, and 14 c tostep up the voltage to a level appropriate for transmission. Thesubstations include various breakers 18 and buses 19, 23, and 25 forproper transmission and distribution of the electric power. The electricpower may be transmitted over long distances using various transmissionlines 20 a, 20 b, and 20 c.

Substations 22 and 35 include step-down transformers 24 a, 24 b, 24 c,and 24 d for stepping down the electric power to a level suitable fordistribution to various loads 30, 32, and 34 using distribution lines26, 28, and 29.

IEDs 102, 104, and 106 are illustrated in substations 16, 22, and 35configured to protect, control, meter and/or automate certain powersystem equipment or devices. According to several embodiments, numerousIEDs are used in each substation; however, for clarity only a single IEDat each substation is illustrated. IEDs 102, 104, and 106 may beconfigured to perform various time dependent tasks including, but notlimited to, monitoring and/or protecting a transmission line,distribution line, and/or a generator. Other IEDs included in asubstation may be configured as bus protection relays, distance relays,communications processors, automation controllers, transformerprotection relays, and the like. As each IED or group of IEDs may beconfigured to communicate on a local area network (LAN) or wide areanetwork (WAN), each IED or group of IEDs may be considered a node in acommunications network.

As discussed above, an IED may be configured to calculate andcommunicate synchrophasors with other IEDs. To accurately comparesynchrophasors obtained by geographically separate IEDs, each IED mayneed to be synchronized with a precision time reference with accuracygreater than a millisecond to allow for time-aligned comparisons.According to various embodiments, time synchronization, accurate to themicrosecond or nanosecond range, may allow IEDs to perform accuratecomparisons of synchrophasors.

FIG. 2 illustrates system 200 configured to be a highly reliable,redundant, and distributed system of time distribution devices 204, 206,and 208 capable of providing a precision time reference to various timedependent IEDs 212, 214, and 216. Each time distribution device 204,206, and 208 may be configured to receive and communicate time signalsthrough multiple protocols and methods. While the system 200 isdescribed as being capable of performing numerous functions and methods,it should be understood that various systems are possible that may haveadditional or fewer capabilities. Specifically, a system 200 mayfunction as desired using only one protocol, or having fewer external orlocal time signal inputs.

As illustrated in FIG. 2, three time distribution devices 204, 206, and208 have WAN capabilities and are communicatively connected to a WAN218, which may comprise one or more physical connections and protocols.Each time distribution device 204, 206, and 208 may also be connected toone or more IEDs within a local network. For example, time distributiondevice 204 is connected to IED 212, time distribution device 206 isconnected to IEDs 214, and time distribution device 208 is connected toIEDs 216. A time distribution device may be located at, for example, apower generation facility, a hub, a substation, a load center, or otherlocation where one or more IEDs are found. In various embodiments, anIED may include a WAN port, and such an IED may be directly connected toWAN 218. IEDs may be connected via WAN 218 or connection 210. Connection210 may be, for example, a local area network (LAN) or a dedicated timecommunication link, such as an Inter-Range Instrumentation Group (IRIG)compliant communication link. In various embodiments, connection 210 mayinclude multiple connections, for example, both a LAN and IRIGconnection. Time distribution devices 204, 206, and 208 may establishand maintain a precision time reference among various system components.Each time distribution device 204, 206, and 208 may be configured tocommunicate time information with IEDs connected on connection 210through one or more time distribution protocols, such as IEEE 1588.

Each time distribution device 204, 206, and 208 is configured to receivetime signals from a variety of time sources. For example, asillustrated, time distribution device 204 includes an antenna 220 and isconfigured to receive a GNSS signal from a GNSS repeater or satellite202. Time distribution device 204 is also configured to receive a secondtime signal 221 from an external time source 201. The external timesource may comprise one or more VCTCXOs, phase locked loop oscillators,time locked loop oscillators, rubidium oscillators, cesium oscillators,NIST broadcasts (e.g., WWV and WWVB), and/or other devices capable ofgenerating precise time signals. In the illustrated embodiment, timedistribution device 208 includes an antenna 220 configured to receive aGNSS signal from the GNSS repeater or satellite 202. As illustrated,time distribution device 206 does not directly receive an external timesignal, however, according to alternative embodiments, any number andvariety of external time signals may be available to any of the timedistribution devices.

According to one embodiment, WAN 218 comprises a SONET configured toembed a precision time reference in a header or overhead portion of aSONET frame during transmission. Alternatively, a precision timereference may be conveyed using any number of time communicationsmethods including IRIG protocols, NTP, SNTP, synchronous transportprotocols (STP), and/or IEEE 1588 protocols. According to variousembodiments, including transmission via SONET, a precision timereference may be separated and protected from the rest of the WANnetwork traffic, thus creating a secure time distributioninfrastructure. Protocols used for inter IED time synchronization may beproprietary, or based on a standard, such as IEEE 1588 Precision TimeProtocol (PTP).

According to various embodiments, time distribution devices 204, 206,and 208 are configured to perform at least one of the methods ofdetecting failure of a time source described herein. System 200 mayutilize a single method or combination of methods, as described herein.

It is of note that even the most precise time signals may exhibit smalldiscrepancies. For example, depending on the length and routing of theGNSS antenna cable, various clocks may exhibit microsecond level timeoffsets. Some of these offsets may be compensated by the user enteringcompensation settings, or may need to be estimated by the timesynchronization network. Estimation may be performed during long periodsof “quiet” operation (i.e., periods with no faults), with the individualsource results stored locally in a nonvolatile storage register.

FIG. 3 illustrates a time distribution device 304, according to oneembodiment. A time distribution device 304 may include more or lessfunctionality than the illustration. For example, time distributiondevice 304 may include an interface for monitoring equipment in anelectric power delivery system in certain embodiments. Accordingly, invarious embodiments time distribution device 304 may be implementedeither as an IED or as a network device. As illustrated, timedistribution device 304 includes a local time source 302 that provides alocal time signal and a time quality module 305 for establishing aprecision time reference. Time distribution device 304 further includesa pair of line ports 312 and 314 for communications with a WAN or LAN.Time information may be shared over a network and may also be fed intothe time quality module 305. Further, time distribution device 304includes a GNSS receiver 310 for receiving a precision time signal, suchas time from a GNSS via a GNSS antenna 320. Time distribution device 304also includes a WWVB receiver 330 for receiving an NIST broadcast, whichcan be used as a precision time signal, via an external antenna 340. Thereceived precision time signal from either source is communicated to thetime quality module 305 for use in determining and distributing theprecision time reference.

Another time source that may be fed to the time quality module 305includes an external time source 306 that may conform to a timedistribution protocol, such as IRIG. The external time source 306 maycommunicate with another time port such as an IRIG input 308.

The various time information from the WAN (from line ports 312 and/or314), GNSS receiver 310, WWVB receiver 330, and IRIG input 308 are inputinto the time quality module 305. In one embodiment, the inputs may befed into a multiplexer (not shown) prior to being input into the timequality module 305. The time quality module 305 functions to determine aprecision time reference for use by the various devices connected to thetime distribution device 304. The precision time reference is thencommunicated from the time quality module 305 to the various devices 322using IRIG protocol (via the IRIG-B output 316) or to various devices325 using another protocol 313 such as IEEE 1588 using Ethernet DropPorts 318. The Ethernet Drop Ports 318 may also include networkcommunications to the various devices connected to time distributiondevice 304. Time distribution device 304 may further include connectionsto SONETs and transmit the precision time reference in a header oroverhead portion of SONET frames.

Time distribution device 304 may also comprise a time signal adjustmentsubsystem 324. Time signal adjustment subsystem 324 may be configured totrack drift rates associated with various external time sources withrespect to local time source 302. Time signal adjustment subsystem 324may also communicate time signals according to a variety of protocols.Such protocols may include inter-Range Instrumentation Group protocols,IEEE 1588, Network Time Protocol, Simple Network Time Protocol,synchronous transport protocol, and the like. In various embodiments,time signal adjustment subsystem 324 may be implemented using aprocessor in communication with a computer-readable storage mediumcontaining machine executable instructions. In other embodiments, timesignal adjustment subsystem 324 may be embodied as hardware, such as anapplication specific integrated circuit or a combination of hardware andsoftware.

According to various embodiments, the time quality module 305 determineswhether a primary or “best available” time source is reliable, i.e., hasnot failed, and distributes the time signal from the best available timesource as the precision time reference to time dependent devices in thesystem. If the best available time source has failed, the time qualitymodule 305 provides an error alert to a user, and in some embodiments,enters a holdover period where an alternative time signal is used forthe precision time reference. In some embodiments, the alternative timesignal is determined by the time quality module and is chosen from theavailable time sources. In another embodiment, the time quality modulecalculates a common time from the available time signals and distributesthe common time to the time dependent devices.

These techniques allow for the best available time source to be used asprecision time reference provided to time dependent devices in a robustmanner such that there is a high likelihood that the precision timereference is accurate. Moreover, in certain embodiments, relying on asecondary time source provided to the time quality module 305 as theprecision time reference during a holdover period when the primary timereference has failed may provide more accurate time information than theholdover situation described above where a local oscillator in each timedependent device is used during the holdover.

FIG. 4 illustrates one embodiment for determining whether a primary orbest available time source has failed. While the time signals in theexample of FIG. 4 are described as specific signals, other signals maybe used with similar results. At 402 the time distribution devicereceives a first time signal from a first time source, or best availabletime source, and provides the time signal to the time quality module. Inone embodiment, the first time source is a time signal received from aGNSS system. GNSS time has the advantages of relying on extremelyaccurate methods for providing the time signal to GNSS receivers, beingreadily available worldwide (particularly in remote locations) 24 hoursper day, and is expected to be stable for many decades to come. GNSSreceivers can keep an internal time, based on the GNSS signal that isaccurate to better than nanoseconds and the time output at the 1 PPSdedicated time port is typically better than 1 microsecond.

At 404 the time distribution device receives a second time signal from asecond time source. In one embodiment, the second time source is a NISTbroadcast such as WWVB. While not as accurate as a time referencederived from a GNSS signal, a time reference derived from a WWVBbroadcast is still very accurate. While the example of FIG. 4specifically uses a WWVB broadcast as the second time source, one ofskill in the art will recognize that other time sources, such as thosedescribed above, can be used in place of the WWVB broadcast.

At 406 the time quality module compares the first time signal to thesecond time signal. Each of the time signals received by the timequality module have an inherent error bound related to the accuracy ofthe time signal. In one embodiment, the time quality module compares thetime signals with regard to their respective error bounds to determinewhether the first time source has failed. For example, given therelatively smaller error bound found in the time derived from a GNSSsignal compared to that found in a time derived from a WWVB broadcast,the time based on the GNSS signal should fall within the error bound ofthe time based on the WWVB broadcast. However, if the GNSS based timesignal falls outside of the error bound of the WWVB based time signal,the time quality module detects, at 408, that there is an error with theGNSS based time signal.

If, at 408, the time quality module determines that the first timesource has not failed, the time quality module distributes time from thefirst time signal as the precision time reference at 410. If, at 408,the time quality module determines that the first time source hasfailed, at 412 the time quality module alerts a user that the bestavailable time source has failed and that the time may not be accurate.In addition to alerting a user of the failure, the time quality moduleat 414 can determine a best available time source and at 416 distributethe best available time source as the precision time reference. Theprocess for determining the best available time source is described inmore detail below with reference to FIG. 7.

While the example of FIG. 4 is limited to a first and second timesignal, the time quality module can continue to compare time signals inorder of relative error bounds beyond just a first and second timesignal. For example, the WWVB based time may be compared to the time ofa local oscillator (taking into account the drift rate of theoscillator) to determine whether the WWVB source has failed, etc.

FIG. 5 illustrates a second embodiment for determining whether a primaryor best available time source has failed. While the time signals in theexample of FIG. 5 are described as specific signals, other signals maybe used with similar results. At 502 the time distribution devicereceives a first time signal from a first time source, or best availabletime source, and provides the time signal to the time quality module. Inone embodiment, the first time source is a time signal received from aGNSS system.

At 504 the time distribution device uses the first time signal to trainan unlocked oscillator to track the time provided in the first timesignal. While the oscillator is trained to track the time of the firsttime source, because the oscillator is unlocked the time provided by thetrained oscillator will drift from that of the first time signal.However, the rate of drift is low and the time distribution devicemaintains the training relationship between the first signal and theoscillator such that the drift is corrected.

At 506 the time quality module compares the first time signal to thetrained oscillator (again, taking into account the drift rate associatedwith the trained oscillator). In one embodiment, a counter tracks thenumber of oscillations of the oscillator between each PPS received fromthe first time signal. Because the oscillator is trained to the firsttime signal, any variation in the oscillation count from PPS to PPSshould be low. If there is a large jump in the variation in theoscillation count, the time quality module, at 508, detects a failure ofthe first time source. The threshold for determining whether the timequality module detects a failure of the time source may depend on thecharacteristics of the oscillator used. For example, a temperaturecompensated crystal oscillator (TCXO) may have a draft rate in theparts-per-million range while oven controlled crystal oscillators andcesium based oscillators may have a drift rate in the parts-per-billion.Thus, the threshold for the more accurate oscillator may be higher. Ifthe variation in the oscillation count exceeds the threshold, the timequality module may indicate a failure of the first time source.

In another embodiment, the oscillator may be used to validate timequality measurements transmitted as part of the time source. Forexample, an IRIG signal includes a Time Quality and Continuous TimeQuality indication. The time quality module may use the oscillator tovalidate the time quality signal received as part of the time source.

If, at 508, the time quality module determines that the first timesource has not failed, the time quality module distributes time from thefirst time signal as the precision time reference at 510. If, at 508,the time quality module determines that the first time source hasfailed, at 512 the time quality module alerts a user that the bestavailable time source has failed and that the time may not be accurate.In addition to alerting a user of the failure, the time quality moduleat 514 can determine a best available time source and at 516 distributethe best available time source as the precision time reference. Theprocess for determining the best available time source is described inmore detail below with reference to FIG. 7.

The example embodiments above provide for a robust system of providing aprecision time reference to time dependent devices by comparing severaltime signals to determine whether the best available time source hasfailed. FIG. 6 illustrates one embodiment for determining whether aprimary or best available time source has failed based on GNSS location.In embodiments where GNSS is the best available time source, thelocation derived from the GNSS signal can be used a check for failure ofthe GNSS time source. This method is particularly useful in embodimentswhere the time distribution device is at a known, fixed location. In oneembodiment, the known location of the time distribution device can beentered by a user at the time of setup and can be modified as necessary.In another embodiment, the known location of the time distributiondevice can be calculated using GNSS signals.

At 602 the time distribution device receives the GNSS signal. While theexample of FIG. 6 is described in terms of a single GNSS signal forclarity, one of ordinary skill in the art will recognize that multiplesignals from various GNSS satellites are typically used in determiningGNSS receiver location and can be used to more accurately calculate GNSSreceiver location. At 604, the GNSS receiver calculates the location ofthe time distribution device based on the received GNSS signal. The timequality module, at 606, compares the calculated location of the timedistribution device with the known location of the time distributiondevice and determines whether the calculated location falls within athreshold distance from the known location. Because GNSS locationcalculation varies based on the techniques employed by the GNSSreceiver, the threshold distance can vary from device to device.

If, at 608, the time quality module determines that the GNSS locationfalls within the threshold, the time quality module distributes the GNSStime as the precision time reference at 610. If, at 608, the timequality module determines that the GNSS location falls outside of thethreshold and therefore the GNSS time source has failed, at 612 the timequality module alerts a user that the best available time source hasfailed and that the time may not be accurate. In addition to alerting auser of the failure, the time quality module at 614 can determine a bestavailable time source and at 616 distribute the best available timesource as the precision time reference. The process for determining thebest available time source is described in more detail below withreference to FIG. 7.

In another embodiment, the time quality module may calculate a locationdrift rate using the GNSS signal and compare the location drift rate toa defined threshold. If the location drift rate exceeds the definedthreshold, the time quality module may determine, at 608, that the GNSStime source has failed.

In one embodiment, the time quality module monitors instantaneous andaverage GNSS signal strength. If the instantaneous signal strength islarger than a set threshold for a set number of samples, then the timequality module may determine that the GNSS time source has failed. Insuch an instance, the time quality module may alert a user and/or relyupon a secondary time signal.

In another embodiment, satellite constellation may be monitored.Satellite constellation repeats every 24 hours. The time quality modulemay determine that the GNSS time source has failed by detecting a changein satellite constellation. In such an instance, the time quality modulemay alert a user and/or rely on a secondary time signal.

FIG. 7 illustrates one embodiment for determining/calculating the bestavailable time source. At 702, the time distribution device receives twoor more consecutive time signals from each of a plurality of timesources. In various embodiments, the times sources may include thosedescribed above. In one embodiment, a time signal may be, for example, apulse-per-second (PPS) signal.

At 704, a time quality module of the time distribution device, maydetermine a duration between each of the two or more consecutive timesignals for each of the plurality of time sources. In one embodiment,the duration between the two or more consecutive time signals isdetermined based on cycles of an internal oscillator. In variousembodiments, the internal oscillator may be a high accuracy oscillatorsuch as an oven controlled crystal oscillator (OCXO), a temperaturecompensated crystal oscillator (TCXO), a voltage controlled crystaloscillator (VCXO), a rubidium oscillator, an atomic oscillator, or thelike.

At 706, the time quality module may compare the duration between the twoor more consecutive time signals for each time source to determine atime source with the lowest variation in the duration between two ormore consecutive time signals. For example, a first time source and asecond time source provide a PPS to the time distribution device. Thetime quality module calculates a duration between consecutive PPS pulsesfor each of the first and second time source. The time quality modulemay then determine a variation of the duration between consecutive PPSpulses.

At 708, the time quality module may select the time source with thelowest variation in duration between consecutive time signals as thebest available time source and at 710 distribute a time signal based onthe best available time source to one or more consuming devices (e.g.,an IED).

At 712, the time quality module may monitor the best available timesource to determine a failure of the best available time source. Thetime quality module may determine the failure through methods such asthose described above with reference to FIGS. 4-6.

At 714, if the time quality module determines that the best availabletime source has failed, the time quality module may determine a backuptime source by returning to 706 to determine the best available timesource. In some embodiments, when the time quality module determines abackup time source, the original best available time source is againincluded in the process at 706 and in some embodiments, the originalbest available time source may be excluded.

FIG. 8 illustrates an example diagram depicting the variation of theduration between two or more consecutive time signals for a particulartime source. The example of FIG. 8 shows a time axis 802 and a freerunning count axis 802. The free running count may be provided by aninternal oscillator as described above. The free running count over theperiod from t0 to t9 is depicted by broken line 806. The example alsoincludes a maximum variation 808 and a minimum variation 812 from theaverage free running count 810.

The above description provides numerous specific details for a thoroughunderstanding of the embodiments described herein. However, those ofskill in the art will recognize that one or more of the specific detailsmay be omitted, or other methods, components, or materials may be used.In some cases, operations are not shown or described in detail.

While specific embodiments and applications of the disclosure have beenillustrated and described, it is to be understood that the disclosure isnot limited to the precise configuration and components disclosedherein. Various modifications, changes, and variations apparent to thoseof skill in the art may be made in the arrangement, operation, anddetails of the methods and systems of the disclosure without departingfrom the spirit and scope of the disclosure.

What is claimed is:
 1. A method comprising: receiving, at a timedistribution device, two or more consecutive time signals from each of aplurality of time sources; determining, for each time source, a durationbetween each of the two or more consecutive time signals, wherein theduration is based on an internal oscillator; comparing the durationbetween the two or more consecutive time signals for each time source todetermine a time source with the lowest variation in the durationbetween the two or more consecutive time signals; selecting the timesource with the lowest variation as a best available time source; anddistributing the best available time source to one or more consumingdevices.
 2. The method of claim 1, wherein the internal oscillatorcomprises a high accuracy oscillator.
 3. The method of claim 2, whereinthe high accuracy oscillator comprises one of: an oven controlledcrystal oscillator (OCXO); a temperature compensated crystal oscillator(TCXO); a voltage controlled crystal oscillator (VCXO); a rubidiumoscillator; or an atomic oscillator.
 4. The method of claim 1, whereinthe plurality of time sources include one or more of: Inter-RangeInstrumentation Group (IRIG) protocol, a global navigation satellitesystem (GNSS), a National Institute of Science and Technology (NIST)broadcast, an Institute of Electrical and Electronics Engineers (IEEE)1588 protocol, a network time protocol (NTP), a simple network timeprotocol (SNTP), or a precision time protocol.
 5. A time distributiondevice comprising: a plurality of receivers configured to receiveconsecutive time signals from each of a plurality of correspondingprecision time sources; an output configured to provide a precision timesignal to an intelligent electronic device (IED), wherein the precisiontime signal is determined from a best available time source of theplurality of corresponding precision time sources; and a time qualitymodule configured to: determine, for each of the plurality of precisiontime sources, a duration between two or more consecutive time signals,wherein the duration is based on an internal oscillator; compare theduration between the two or more consecutive time signals for each timesource to determine a time source with the lowest variation in theduration between the two or more consecutive time signals; and selectthe precision time source with the lowest variation as the bestavailable time source.
 6. The time distribution device of claim 5,wherein the internal oscillator comprises a high accuracy oscillator. 7.The time distribution device of claim 6, wherein the high accuracyoscillator comprises one of: an oven controlled crystal oscillator(OCXO); a temperature compensated crystal oscillator (TCXO); a voltagecontrolled crystal oscillator (VCXO); a rubidium oscillator; or anatomic oscillator.
 8. The time distribution device of claim 5, whereinthe plurality of precision time sources include one or more of:Inter-Range Instrumentation Group (IRIG) protocol, a global navigationsatellite system (GNSS), a National Institute of Science and Technology(NIST) broadcast, an Institute of Electrical and Electronics Engineers(IEEE) 1588 protocol, a network time protocol (NTP), a simple networktime protocol (SNTP), or a precision time protocol.
 9. The timedistribution device of claim 5, wherein the time quality module isfurther configured to: detect a failure of the selected precision timesource; and determine a backup precision time source for use as the bestavailable time source, wherein the process to determine the bestavailable time source is repeated to determine the backup time source.10. The time distribution device of claim 9, wherein to detect a failureof the selected precision time source the time quality module isconfigured to: train an unlocked oscillator with a time signal from thebest available time source; compare the time signal from the bestavailable time source to a drift rate of the unlocked oscillator; anddetect a failure of the best available time source in response to thecomparing showing that the time signal from the best available timesource varies from the drift rate of the unlocked oscillator by adefined margin.
 11. The time distribution device of claim 9, wherein thetime quality module is further configured to indicate an error conditionin response to detecting a failure of the best available time source.12. The time distribution device of claim 9, wherein to detect a failureof the selected precision time source the time quality module isconfigured to: receive a location based on a plurality of GlobalNavigational Satellite System (GNSS) signals received at the timedistribution device; compare the received location with a known locationof the time distribution device; and detect a failure of the bestavailable time source in response to the received location and the knownlocation differing by a defined margin.
 13. The time distribution deviceof claim 12, wherein the known location is calculated using theplurality of GNSS signals.
 14. The time distribution device of claim 12,wherein the known location is entered into the time distribution deviceat setup.
 15. A non-transitory computer readable storage medium havinginstructions stored thereon, which when executed by a processor, causethe processor to perform a method for determining a best available timesource, the method comprising: receiving, at a time distribution device,two or more consecutive time signals from each of a plurality of timesources; determining, for each time source, a duration between each ofthe two or more consecutive time signals, wherein the duration is basedon an internal oscillator; comparing the duration between the two ormore consecutive time signals for each time source to determine a timesource with the lowest variation in the duration between the two or moreconsecutive time signals; and selecting the time source with the lowestvariation as the best available time source.
 16. The non-transitorycomputer readable storage medium of claim 15, wherein the internaloscillator comprises a high accuracy oscillator.
 17. The non-transitorycomputer readable storage medium of claim 16, wherein the high accuracyoscillator comprises one of: an oven controlled crystal oscillator(OCXO); a temperature compensated crystal oscillator (TCXO); a voltagecontrolled crystal oscillator (VCXO); a rubidium oscillator; or anatomic oscillator.
 18. The non-transitory computer readable storagemedium of claim 15, wherein the plurality of precision time sourcesinclude one or more of: Inter-Range Instrumentation Group (IRIG)protocol, a global navigation satellite system (GNSS), a NationalInstitute of Science and Technology (NIST) broadcast, an Institute ofElectrical and Electronics Engineers (IEEE) 1588 protocol, a networktime protocol (NTP), a simple network time protocol (SNTP), or aprecision time protocol.
 19. The non-transitory computer readablestorage medium of claim 15, wherein the method further comprisesdistributing the best available time source to one or more consumingdevices.
 20. The non-transitory computer readable storage medium ofclaim 15, wherein the method further comprises: detecting a failure ofthe selected time source; and determining a backup time source for useas the best available time source, wherein the method for determiningthe best available time source is repeated to determine the backup timesource.